The PIN code is a six-digit number which is used to encrypt the private key of a Burstcoin account. The PIN code does not get saved on your device. You have to enter it everytime you want to do a transaction. This prevents people who got access to your phone from unauthorized transfer of Burstcoins to other addresses. Therefore, the PIN is an additional security mechanism to keep your Burstcoins safe.
If you have forgotten or lost your PIN code, just remove the corresponding account and import it again by entering its passphrase. This way you are allowed to set a new PIN code for further transactions.